Signal translating system



H. M. 1 Ewls SIGNAL TRANSLATING SYSTEM Jap. 14, 1941.

Filmed Juneza. 1958 .NGE

menno amd ATTORNEY Patented Jan. 14, 1941 UNITED STATES PATENT OFFICE SIGNAL TRAN SLATIN G ySYSTEM Ware Application June 28, 1938, Serial No. 216,200

12 Claims.

This invention relates to television systems and more particularly to improved television videofrequency signal-translating systems.

In accordance with present television practice,

a transmitted signal comprises pictureor videofrequency components representative of varying light values in successive areas of an image being transmitted and synchronizing-signal components which correspond to initiations of successive lines and elds in the scanning of the image. The video-frequency components occur during the trace-scanning periods, Vwhile the synchronizing-signal components occur during the retracescanning periods. Between the trace-scanning periods, thev signal is blocked out, that is, it includes block-out components which ordinarily have amplitude values extending outside of the amplitude range of the video-frequency components on the black side of the axis of the modulation wave. There is usually also developed at the transmitter va unidirectional voltage which corresponds to the average background illumination of the scene. While this voltage is lost when the signal is passed through alternating current ampliers, the signal at all times includesa com,- ponent from which the unidirectional voltage representative of background illumination may be derived.

There is provided at the receiver a lsignal-reproducing device, such as a cathode-ray tube, in which a scanning beam is deflected to scan, in series of parallel lines or fields, a fluorescent screen upon which the image of the transmitted scene is reconstructed. The synchronizing-signal components of the received signal are utilized to control the scanning apparatus at the receiver so as to synchronize its operation with that of similar apparatus at the transmitter, while the intensity. of the cathode ray is controlled by the video-frequency signal components, as well as the unidirectional background-illumination voltage, thereby to reconstruct the scene.

One 'of the essential functions which must be performed in a television system is the establishment of a fixed signal level corresponding to black. Due to the fact that the block-out components of the signal ordinarily extend substantially beyond the amplitude range of the videofrequency signal on the black side of the signal axis, the peak values of these block-out components do not correspond to the black level of the signal unless the amplitude of such components is limited; `that is, unless portions thereof beyond .the black level are suppressed. Moreover, it has been found diflicult to limit such a television signal including such components by devices hereto-fore utilized Without substantially distorting the wave formof the signal. In general, the grid-voltage plate current characteristic of a tube designed to operate over a wide grid swing, that is, to accommodate a relatively large input-signal amplitude, has a curvature near cuto which is very gradual relative to that of the corresponding characteristic of a tube designed to operate over a small grid swing. In the latter type of tube, however, this range of curvature bears the `same relationship `to the normal amplitude range of the input signal as in the case of the first-mentioned tube. Hence, the same percentage of signal distortion results from the use of either type of tube if the signal amplitude is adjusted to a range corresponding to the normal operating range of the tube. However, if a large amplitude signal is applied to a tube designed for a relatively small grid swing, the tube has a relatively sharp characteristic with respect to the large amplitude signal, but it can repeat only a small part of the signal.

It is an object of the present invention, therefore, to provide an improved television videofrequency signal-translating system, whereby portions of the signal beyond a predetermined value on one side of its axis may be substantially suppressed without distorting the resultant signal.

A further object of the invention is .to provide a system of .the character described, whereby the black level of the signal may be readily set without distortion of the useful porti-ons of the signal.

In accordance with the invention, a television video-frequency signal-translating system for suppressing portions ofa translated video-frequency signal of lesser amplitude values than that corresponding to` a predetermined shade value thereof comprises a first channel including 40 an output circuit for translating the signal with one polarity, together with means for applying to the channel the video-frequency signal including background components. This first channel includes means for providing a stabilized signal in its output circuit. A second channel is provided including an output circuit for translating the signal with opposite polarity and means are provided for applying the above-mentioned videofrequency signal to the second channel. The second channel also includes means for providing a stabilized signal of opposite polarity in its output circuit and limiting means for passing `only portions of the translated signal of lesser amplitude value than that corresponding to the P Il above-mentioned shade value. Means are provided for combining the oppositely-poled unidirectional signal outputs of the above-mentioned channels including the background components of .the `translated signals, whereby the lesser amplitude value portions of the translated signal are at least partially suppressed. For example, the portion of the signal beyond the value on one side of its axis corresponding to black may be suppressed so as to establish a predetermined black level for the signal, that is, to set the tone of black.

Moreover, the signal to be translated may include background-illumination components as well as .the video-frequency components, and these components may be represented by the peak value of the signal on o-ne side of its axis; for example, the peak value of the block-out signal components on the black side of the axis. In such a case, in accordance with the present invention, there may be provided rectifying means for deriving from the signal a background-illumination control-bias voltage representative of the background illumination, that is, proportional to the said peak value of the signal, and means for applying the derived bias voltage to the two channels of the system, thereby to stabilize the signal inputs thereto.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is acircuit diagram, partially schematic, of a television transmitting system including circuits embodying the present invention, while Fig. 2 is a graph representing certain operating characteristics of the system of Fig. 1, to aid in the understanding ofthe invention.

Referring now more particularly to Fig. 1 of the drawing, there is illustrated a television transmitting system comprising a signal generator I0, which may be of conventional design and include the usual cathode-ray signal-generating tube and scanning elements. For the purpose of developing scanning voltages or currents for the generator I0, there are provided a line-frequency saw-tooth wave generator II and a field-frequency saw-tooth wave generator I2, the output circuits of these generators being connected to the scanning elements of the signal generator I in the usual manner. In order to block out the scanning ray of the generator I0 during retrace scanning periods, there is provided a blockout wave generator I3 having its output circuit suitably connected to the generator I0. For providing impulses to suppress undesirable signal (ill impulses during retrace scanning periods and to ensure proper form of the modulation signal to be developed, there is provided a pedestal-impulse generator I4. A synchronizing signal generator I5 is also provided for developing synchronizingsignal components. y

\ For the purpose of synchronizing the generators II-I5, inclusive, there is provided a timing-impulse generator I6 coupled to the input circuits of the generators II-I5, inclusive. The timing-impulse generator I6 is preferably stabilized by means of a connection I1 to a suitable source of periodic voltage, for example, the power supply circuit or the synchronizingvoltage source of a motion picture mechanism, where such is employed.

Connected in cascade to the output circuit of the signal generator III, in the order named, are a first video-frequency amplifier IB, a second video-frequency amplier I9, which embodies the present invention and will be hereinafter described in more detail, a third video-frequency amplifier 20, a modulator 2I and associated carrier-frequency oscillator 22, a power amplifier 23, and an antenna system 24, 25, all constructed and arranged according to conventional practice. The output circuit of the pedestal generator I4 is coupled to the video-frequency amplifier I3, while the output circuit of the synchronizing-signal generator I5 is coupled to the video-frequency amplier 20. Suitable means (not shown but which may be considered as included in the signal generator IIJ) are also provided for developing a unidirectional background-illumination voltage and applying it to the amplifier I8.

Neglecting for the moment the details of operation of the video-frequency ampliiier IB, the system just described comprises the elements of a television transmission s-ystem of conventional design and, the various parts thereof being illustrated schematically and being of any Well-known construction, a detailed description of its operation is unnecessary. Briefly, however, the image of a scene to be transmitted is focused on the target of the signal generator l0. Scanning currents or voltages are applied to the scanning elements of the generator I0 to supply electric fields which serve to deflect the scanning ray horizontally and vertically, thereby to scan successive series of parallel lines, or'fields, upon the target. The deflecting currents or voltages and, hence, the scanning elds are of the well-known sawtooth wave form, providing relatively slow linear trace and rapid retrace scansions. Block-out impulses developed by the generator I3 are applied to the signal` generator II) to suppress or block out, the scanning beam during retrace portions of the scanning cycles, while pedestal impulses developed by the generator I4 are applied to the ampliner i8 to suppress surges which occur in the signal. The timing impulses developed by the generator I6 are applied to the generators II-I5, inclusive, to lock these generators in synchronism.

The photosensitive elements of the target of the signal generator I0 are electrically aiected to an extent dependent upon the varying values of light at the corresponding incremental areas of the image focused thereon so that, as the scanning beam scans the target, a video-frequency voltage of correspondingly varying amplitude is developed in the output circuit of the generator I0 and applied to the video-frequency amplier I8. The unidirectional background-illumination voltage, which is developed by suitable means included in the generator I0, is also applied to the amplifier I8, as well as the pedestal impulses developed by the generator I4. Thus, the Video-frequency components, blockout components, unidirectional background components, and pedestal impulses are all amplified and mixed in the amplifier i8 and are thereupon applied to the video-frequency amplier I9, wherein they are translated in accordance with the present invention and from which they are applied to the video-frequency amplifier 2D. The synchronizing impulses developed by the generator I5 are mixed with the modulation signalin the amplifier 20 and .the resultant modulation signal is thereupon supplied to the modulator 2 I, wherein it is impressed upon the carrierA wave generated by the'oscillator 22.

The modulated-carrier signal is then delivered to the power amplifier 23 for amplication and is impressed upon the antenna system 24, 25 to be broadcast.

Referring now more particularly to the ampliier I9 embodying the present invention, for the purpose of establishing a fixed predetermined black level for the modulation signal, this amplifier comprises a plurality of vacuum amplifier tubes 26, 21, and 28, which are preferably of the pentode type, having high internal impedance. The tubes 26 and 21 are connected in cascade to provide a first signal-translating channel, -While the tube 28 provides a second signal-translating channel connected in parallel with the rst channel, the two channels having substantially equal mutual conductance characteristics.

'Ihe signal input grid of the tube 26 is coupled to the output circuit of the video-frequency amplifier I8 by means of a coupling condenser 29 and leak resistor 30, while the anode circuit of this tube includes a load resistor 3|. The signalinput grid-cathode circuit of the tube 21 is, in turn, connected across the resistor 3| by Way of a coupling condenser 32 and leak resistor 33. The signal-input grid of the tube 28 is connected directly in parallel with that of tube 26. The anode circuits of the tubes 21 and 28 are connected in parallel and include a common load resistor 34 and choke 35 across which the input circuit of the amplifier 20 is connected, as shown. Operating potentials for the tubes 26, 21, and 28 are supplied to their screens from suitable sources indicated at -l-Sc, and to their anodes by way of their load circuits from suitable sources indicated at +B. A source of adjustable-bias voltage is provided for the tube 28, for example, by means of a battery 40 included in its cathode circuit.

For the purpose of reinserting the unidirectional or background-illumination component of the signal, which is lost in transmission through alternating current couplings, such as the condensers 29 and 32, that is, for stabilizing the translated signals at a level corresponding to a predetermined shade value thereof, there are provided diode rectiers 36 and 31 connected with opposite polarities across the input circuits of the ampliers 2B and 28 and the amplifier 21, respectively. and in parallel with the leak resistors 3D and 33, the rectiers 36 and 31 thus comprising means for deriving from the translated signal a background-illumination control voltage proportional to the peak value of the signal on one side of its alternating current axis. The diodes 36 and 31 are also connected in series with sources of suitable biasing potential, indicated by the batteries 38 and 39, respectively. y

Referring now to the operation of the system of the present invention, it will rst be noted that the peaks of the block-out portions of the waves, occurring during retrace-scanning periods and having lesser amplitude values than the video-frequency components thereof, as applied to the tubes of the amplifier I9 are at all times at the same level, that is, the signals are stabilized. This is due to the fact that the background illumination or unidirectional component of the signal is reinstated by the action of the diodes 36 and 31. More particularly, in the absence of a signal the batteries 38 and 40 serve to bias the tube 28 near cutoff, while the battery 39 biases the tube 21 near its cutoi point. 'I'he diodes 36 and 31 derive from the signal outputs of the amplifiers I8 and 26, respectively, negative and positive unidirectional voltages, `respectively, equal to the peak values of the signalen the black side of its axis, which voltages appear across the leak resistors 3Il'andl 33, which also comprise the rectifierv load resistors. These unidirectional voltages are thus applied to the control grids of the tubes 26 and 28 and tube 21 in opposition to the iixed biases from the batteries 38 and 39, respectively. The resultant video-frequency signals applied to the control grids of the tubes, therefore, are of a stabilied wave form including the unidirectional background-illumination components as well as the fvideo-frequency or high-frequency components with the negative peaks of the signaLas stated above, all at the same level. The tube 26 simply serves to reverse the polarity of the signal so that it is applied in opposite senses to the tubes 21'and 28.

The details of operation of the system of the present invention may best be explained with reference to the curves of Fig. 2. In this iigure, the abscissae represent grid voltages and the ordinates, plate current. Curve A represents the grid-voltage plate current characteristic of the tube 21,'the signal voltage as applied to this tub-e being illustrated by curve Ag and having the lowest amplitude value of the arnplitude` lrange of the video-frequency components corresponding to black. Curve B,v on the other hand, represents the similar characteristic of the tube 28, while the signal voltage as applied tothis tubeis also illustrated by curve Ag, its abscissae scale being displaced and running in an opposite sense relative to that of curve A to account for the reversal of polarity of the signal input to the tubes 21 and 28. 'I'he curves are plotted in this manner so as more clearly to illustrate'the lconditions resulting from reversing the polarities of the signals, as applied to the tubes 21 and 28. In curve Ag the portions indicated at v represent the video-frequency components, and the portions indicated at b represent the blockout components.

It will be noted from the curves that the tube 21 is designed to operate with a relatively Wide grid swing and is biased so as to operate within its cutoff ,limits and primarily over the linear portion of its characteristic, thereby to develop in its output circuit a signal ofwhich the video-frequency components are undistorted. The tube 28, on theother hand, is designed to operate over .a relatively narrow grid swing and has a relatively sharper cutoif and is biased well below its cutoff point so that only a portion of the applied vsignal beyond a predetermined value on the side of its axis corresponding to black is repeated in its outputcircuit.

The effective grid-voltage plate current char- 'acteristicof the combination of tubes 21 and 28, connected as shovvnand with the signal applied to their grids with opposite polarities, is represented by the curve C. From this curve it Will be seen that, in the vicinity of the block-out components bjthe slope of the curve, that is, the transconductance of the combination, is substantially zero `so that the block-out components are not rethe value corresponding to black, on the black side of the axis of the signal is substantially suppressed, while the video-frequency portion which is repeated is substantially undistorted. In other words, the black level for the signal is set without distortion of the useful portions of the signal.

More clearly to illustrate the advantages achieved by the arrangement of the present invention, curve D, shown at the left of Fig. 2, represents the output current as it would appear if `the limiting action Were obtained Without utilizing tube 28; that is, simply by increasing the negative bias of the tube 2l so as to operate it beyond cutoff, as indicated by curve Bg. In this case the wave form of the signal output is substantially distorted as indicated at the portions d.

In certain instances it may be desirable to limit the signal above the block-out portions so as to set the black level at a different value, thereby to set the black level of the combined signal at any desired tone value of the signal. For the purpose of adjusting the black level or tone of black in the signal, as developed by the system of the present invention, it is only necessary to adjust the bias on the tube 28, for example, by adjusting the battery 40. Curve C1 illustrates the effective grid-voltage plate current characteristic for the combination for an increase in the negative bias of the tube 28 and the curve C01 illustrates the resultant signal output of tubes 2l and 28 for this condition.

In summary, therefore, it is seen that the television video-frequency signal-translating system of Fig. 1 suppresses the blacker-than-black pori tions of a translated video-frequency. signal, these portions being of lesser amplitude values than a predetermined shade value of the signal, that is, the black level thereof. The system comprises a rst channel including tubes 26 and 21 for translating with one polarity a signal including videofrequency and background components and for providing a stabilized signal in the output circuit thereof. The signal also comprises a second channel, -including tube 28, for translating the v same signal with a polarity opposite to that of the first channel and for providing a stabilized signal at the output circuit thereof. As used in this specification, the phrase translating with opposite polarity means that, for signal inputs to the two channels of the same polarity, signal outputs of different polarities are obtained from the respective channels. It is also seen that a limiting means, specifically, tube 28 biased by battery 40, is included in the second channel for passing only portions of the signal of lesser amplitude value than that of the predetermined shade value, that is, for passing only the blacker-than-black portions of the signal. Means are also provided for combining the oppositely-poled signal outputs of the channel so that the blacker-than-black portions of the signal, or the said portions of lesser amplitude values, are at least partially suppressed.

While there has been described what is at present considered to be the preferred embodiment of this invention, it Will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

l. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that `corresponding to a predetermined shade value thereof comprising, a first 4channel including an output circuit for translating a signal With one polarity, means for applying to said channel a video-frequency signal including background components, said channel including means for providing a stabilized signal in said output circuit, a second channel including an output circuit for translating said signal with opposite polarity, means for applying said videofrequency signal to said second channel, said second channel including means for providing a stabilized signal of opposite polarity in its output circuit and limiting means for passing only portions of said signal of lesser amplitude value than that corresponding to said predetermined shade value, and means for combining the oppositelypoled unidirectional signal outputs of said channels including background components, whereby said lesser amplitude value portions of said signal are at least partially suppressed.

2. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that corresponding to a predetermined shade value thereof comprising, a first channel including an output circuit for translating a signal with one polarity, means for applying to said channel a video-frequency signal including background components, said channel including means for providing a stabilized signal in said output circuit, a second channel including an output circuit for translating said signal with opposite polarity, means for applying said video-frequency signal to said second channel, said second channel including means for providing a stabilized fsignal of opposite polarity in its output circuit and limiting means for passing only portions of said signal of lesser amplitude value than that corresponding toi said predetermined shade value, said tvvo channels having substantially equal mutual conductance characteristics, and means for combining the oppositely-poled unidirectional signal outputs of said channels including background components, whereby said lesser amplitude value portions of said sign-al are at least partially suppressed.

3. A television video-frequency signal-translating system for suppressing portions of a tr-anslated video-frequency signal of lesser amplitude values than that corresponding to a predetermined shade value thereof comprising, a first amplifier including an output circuit for translating a signal with one polarity, means for applying to said amplifier a video-frequency signal including background components, said ampliiier including means for providing a stabilized signal in said output circuit, a second amplifier including anoutput circuit for translating said signal with opposite polarity, means for applying said video-frequency signal to said second amplifier, said second amplifier including means fo-r providing a stabilized signal of opposite polarity in its output circuit and limiting means for passing only portions of said signal of lesser amplitude value than that corresponding to said predetermined shade value, and means for combining the oppositely-poled unidirectional signal outputs of said channels including background components, whereby said lesser amplitude value portions of said signal are at least partially suppressed.

4. A television video-frequency signal-trans lating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that corresponding to .a predetermined shade value thereof comprising, a rst channel including an output circuit for translating a signal with one polarity, means for applying to said channel a video-frequency signal including background components, said channel including means for providing a stabilized signal in said output circuit, asecond channel including an output circuit for translating said signal with opposite polarity, means for applying said video-frequency signal to said second. channel, said second channel including means for providing a stabilized signal of opposite polarity in its output circuit and limitingmeans for passing only portions of said signal of lesser amplitude value than that corresponding tc said predetermined shade value, means for adjusting said limiting means to adjust said predetermined value, and means for combining the oppositelypoled unidirectional signal outputs of said channels including background components, whereby said lesser amplitude value portions of said signal are at least partially suppressed.

5. A television video-frequency signal-trans- A lating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than a predetermined shade value thereof comprising a first channel having a given cutoff characteristic for transl-ating with one polarity a unidirectional video frequency signal including background components, and a second channel for translating said signal with opposite polarity and having a relatively sharper cutoff characteristic and including limiting means for passing only portions of said signal of lesser amplitude value than said predetermined shade value, means for stabilizing the signal inputs to said channels, and means for combining the oppositely poled signal outputs of said channels, whereby said lesser Avalue portions of said signal are at least partially suppressed.

6. A television video-frequency signal-translating system for suppressing portions of a translated Video-frequency signal of lesser amplitude values than a predetermined shade value thereof comprising a rst channel for translating with one polarity a signal including video-frequency and' background-illumination components, a second channel for translating said signal with opposite polarity and including limiting means for passing only portions of said signal of lesser amplitude value than said predetermined shade value, means for deriving from said backgroundillumination components of said signal a unidirection-al-bias voltage representative of background illumination, means for utilizing said bias voltage to stabilize the signal inputs to said channels, and means for combining the oppositely poled signal outputs of said channels, whereby said lesser value portions of said signal are at least partially suppressed.

7. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than a predetermined shade value thereof comprising a first channel for translating with one polarity a signal including video-frequency and background-illumination components represented by the peak value of the signal on one side of its axis, a second channel for translating said signal with opposite polarity and including limiting means for passing only portions of said signal of lesser amplitude value than said predetermined shade value, rectifying means for deriving from said signal a background-illumination control-bias voltage proportional to said peak valueA of said signal, means for applying said bias voltage to said channels to stabilize the signals passed thereby, and means for combining the oppositely poled signal outputs of said channels, whereby said lesser Value portions o-f lsaid signal are at least partially suppressed.

8. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than a predetermined shade value thereof comprising a first channel fortranslating with' one polarity a signal including video-frequency and background-illumination components represented by the peak value of the signal on one side of its axis, afsecond channel for translating said signal with opposite polarity and including limiting means for passing only portions of said -signal of lesser amplitude value than said predetermined shade value, rectifying means for deriving from said signal a background-illumination control-bias voltage proportional to said peak value of said signal, means for applying saidvbias voltage to said channels to stabilize the signals passed thereby, and means for combining the oppositely poled signal outputs of said channels, whereby said lesser value portions of said signal' are atleast partially suppressed, and means fo-r adjusting said limiting means to adjust said predetermined value, thereby to set the black level of the combined signal at any desired tone value.

9. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that correspo-nding to a predetermined shade value thereof comprising.. a iirst channel including an output circuit for translating a signal With 'one polarity, means for applying to said channel a signal including video-frequency components occurring during predetermined trace-scanning periods and block-out components occurring during retracescanning periods and having lesser amplitude values than said video-frequency components,

said channel including means for providing a stabilized signal in said output circuit, a second channel including an output circuit for transf lating said signal with opposite polarity, means for applying said video-frequency signal to said second channel, said second channel including means for providing a stabilizing signal of opposite polarity in its output circuit and limiting means for passing only portions of said signal of lesser amplitude values than that corresponding to said video-frequency components, and means for combining the oppositely-poled unidirectional signal outputs of said channels including background components', whereby said lesser amplitude value portions of said signal are at least partially suppressed. f

10. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that corresponding to a predetermined shade value thereof comprising, a iirst channel including an output circuit for translating a signal with one polarity, means for yapplying to said channel a signal including video-frequency components occurring during' including means for providing a stabilized Signal in said output circuit, a second channel including an output circuit for translating said signal with opposite polarity, means for apply-` ing said video-frequency signal to said second channel, said second channel including means for providing a stabilized signal of opposite polarity in its output circuit and limiting means for passing only` portions of said signal of lesser amplitude values than that corresponding to black, and means for combining the oppositelypoled unidirectional signal outputs of said channels including background components, whereby said portions of said signal of lesser amplitude values than said value corresponding to black are at least partially suppressed to establish a predetermined black level for said signal.

11. A television video-frequency signal-translating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that corresponding to a predetermined shade value thereof comprising, a first channel including an output circuit for translating a signal with one polarity, means for applying to said channel a signal including video-frequency components occurring during trace-scanning periods and block-out components occurring during retrace-scanning periods and having lesser amplitude values than the amplitude values of said video-frequency components corresponding to black, said channel including means for providing a stabilized signal in said output circuit, a Vsecond. channel including an output circuit for translating said signal with opposite polarity, means for applying said signal to said second channel, said second channel including means4 for providing a stabilizedl signal of opposite polarity in its output; circuit and limiting means for passing only portions of said signal of lesser amplitude values than that corresponding to said predetermined value corresponding to black, means for combining the oppositely-poled unidirectional signal outputs of said channels including background components, whereby said portions of said signal of lesser -amplitude values than said value corresponding to black are at least partially suppressed to establish a predetermined black level for said signal, and means for adjusting said limiting means to adjust said predetermined value, thereby to set the black level of the combined signalV at any desired tone value of the signal.

12. A televisiony video-frequency signal-trans lating system for suppressing portions of a translated video-frequency signal of lesser amplitude values than that corresponding to. a predetermined shade value thereof comprising, a rst channel including an output circuit for translating a signal,v means for applying to said channel said signal With such polarity that an increase in signal-amplitude value corresponds to a decrease in shade values, said channel including means for providing a stabilized signal in said output circuit, a second channel including an output circuitv for translating said signal with opposite polarity, means for applying said videofrequency signal to said second channel, said second channel including means for providing a stabilized signal of opposite polarity in its output circuit and limiting means for passing only portions of said signal of lesser amplitude values than that corresponding to said predetermined shade value, and means for combining the oppositely-poled unidirectional signal outputs of said channels including backgroundV components, whereby said lesser amplitude value portions of said signal are at least partially suppressed.

HAROLD M. LEWIS. 

